1. Field of the Invention
This invention relates to mixed-mode/mixed-signal integrated circuit fabrication technology, and more specifically to a vertically stacked high electron mobility transistor (HEMT)-heterojunction bipolar transistor (HBT) integrated circuit and method of fabrication.
2. Description of the Related Art
Mixed-mode or mixed-signal devices take advantage of the different characteristics of field effect transistors (FETs) and bipolar transistors to achieve circuit functions and performance levels that are not currently achievable with existing single-device technologies. HEMTs have very low noise and high current gain characteristics which make them well suited for detecting faint signals. However, HEMTs are highly nonlinear and thus cannot operate over a large dynamic range. On the other hand, HBTs are relatively noisy but highly linear, and thus are well suited for the amplification of large signals. As a result, the two types of devices can be combined to form a low noise amplifier (LNA), with the HEMT providing the front end receiver and the HBT a highly linear output amplifier stage. Furthermore, known analog or digital bipolar circuit configurations can be used to control a HEMT amplifier. For example, an HBT circuit can be used to regulate the current in the HEMT amplifier.
One known approach is to fabricate separate HEMT and HBT integrated circuits that perform respective amplification and biasing functions, bond them to a carrier, and then interconnect their pins using conventional wire bonding techniques to produce the desired mixed-mode/mixed-signal circuit configuration. The advantage of this approach is that the fabrication of single-device integrated circuits is well known. However, the mixed-mode/mixed-signal circuit is not a monolithic integrated circuit, with its attendant cost and performance advantages. Wire bonding of the separate single-device circuits is labor intensive and expensive, and requires a large spacing between the devices. Moreover, it adds resistance to the circuit which lowers its speed, and unshaped wires produce undesirable capacitive and/or inductive effects and impedance matching problems at microwave and millimeter wave frequencies.
Streit et al., "Monolithic HEMT-HBT Integration by Selective MBE," IEEE Transactions on Electron Devices, vol. 42, No. 4, pages 618-623, 1995 discloses a selective growth process for integrating a Schottky-gate HEMT and an HBT in a monolithic microwave integrated circuit (MMIC). First, a GaAs substrate is placed in a chamber and a multi-layer npn-HBT structure is grown on the substrate using a molecular beam epitaxy (MBE) process. The substrate is removed from the chamber and patterned to roughly define the HBT device. The patterning process includes depositing a layer of silicon nitride on the HBT structure, depositing a mask over the silicon nitride, and plasma etching the exposed silicon nitride with chlorine-fluoride gas to selectively remove the silicon nitride and define the HBT device. The silicon nitride patterning process is expensive and time consuming.
The substrate is then put back into the chamber for a second MBE growth to produce the multi-layer HEMT structure, followed by etching the substrate with hydrofluoric acid to remove the remaining silicon nitride from the HBT structure. The compositions of the multi-layer HBT and HEMT structures are selected to optimize their respective performances. Thus, the materials and thicknesses of the individual layers are not the same. Once both the HBT and HEMT structures are formed, they are patterned and metallized using conventional etching and deposition processes, respectively, to define the devices' active areas and metal contacts. This includes a trench etch through the HEMT structures to define their Schottky barriers. The trench etch is difficult to control, and thus reduces the HEMT uniformity across the wafer.
Although Streit's HEMT-HBT is integrated, and thus realizes the advantages of integrated circuits, it has a number of serious deficiencies. The growth and regrowth of the HBT and HEMT structures increases fabrication time, which increases the cost of the IC. When the substrate is removed from the chamber between growths for processing it can become contaminated, which reduces the quality of the HEMT material grown in the regrowth stage. Furthermore, during regrowth the HEMT material tends to build up along the edge adjacent the HBT structure. As a result, the periphery of the HEMT can be of poor quality and unusable as a part of the HEMT's active area. This increases the spacing between devices, which reduces the number of devices that can be fabricated on a wafer. The regrowth process also exposes the device to temperatures in excess of 600.degree. C., which causes dopants in the highly doped base region to diffuse into the emitter and collector regions, thereby reducing the abruptness of the pn junctions and lowering the HBT's current gain.
Streit's growth and regrowth process produces HEMT and HBT devices that are non-planar, i.e., they have large step discontinuities between their metal contacts. This occurs because the two devices are fabricated in independent growths and thus cannot be exactly matched, they have different multi-layer structures to optimize their performances, and the HEMT structure builds up near the HBT during regrowth. As a result, the potential for gaps in the metal interconnections formed through conventional deposition processes is high and the chance that metal will be deposited where it is not wanted is also high. This reduces the reliability of the HEMT-HBT device. Furthermore, thicker metal interconnections are required to reduce the chance of breakage due to the step discontinuities, which increases the weight of the device.
Zampardi et al. "Circuit Demonstrations in GaAs BiFET Technology," Solid-State Electronics, Vol. 38, No. 9, pp. 1723-1726, 1995 disclose a Schottky-gate MESFET-HBT integrated circuit. The MESFET and HBT share only a single layer: the MESFET's channel and the HBT's emitter. In order to provide the necessary HBT emitter characteristics, Zampardi uses aluminum gallium arsenide (AlGaAs), which provides poor channel mobility for the MESFET.
Usagawa et al., (fill in title) IEDM, pp 78- (fill in page range), 1987 discloses an integrated pnp-HBT/HEMT in which the devices share a single layer: the HEMT's channel and the HBT's base. The shared layer includes a two dimensional electron gas that is necessary to provide the high electron mobility in the HEMT. The gas layer is very thin, and thus the HBT's base is susceptible to "punch-through" when reverse biased under a large electric field. This can destroy the HBT's bipolar operation. Furthermore, the pnp-HBT is very slow, due to the low mobility of holes with respect to that of electrons.